The Common Lisp Cookbook - Input/Output


Redirecting the Standard Output of your Program

You do it like this:
(let ((*standard-output* <some form generating a stream>))
Because *STANDARD-OUTPUT* is a dynamic variable, all references to it during execution of the body of the LET form refer to the stream that you bound it to. After exiting the LET form, the old value of *STANDARD-OUTPUT* is restored, no matter if the exit was by normal execution, a RETURN-FROM leaving the whole function, an exception, or what-have-you. (This is, incidentally, why global variables lose much of their brokenness in Common Lisp compared to other languages: since they can be bound for the execution of a specific form without the risk of losing their former value after the form has finished, their use is quite safe; they act much like additional parameters that are passed to every function.)

If the output of the program should go to a file, you can do the following:

(with-open-file (*standard-output* "somefile.dat" :direction :output
                                   :if-exists :supersede)
WITH-OPEN-FILE opens the file - creating it if necessary - binds *STANDARD-OUTPUT*, executes its body, closes the file, and restores *STANDARD-OUTPUT* to its former value. It doesn't get more comfortable than this!

Faithful Output with Character Streams

By faithful output I mean that characters with codes between 0 and 255 will be written out as is. It means, that I can (PRINC (CODE-CHAR 0..255) s) to a stream and expect 8-bit bytes to be written out, which is not obvious in the times of Unicode and 16 or 32 bit character representations. It does not require that the characters ä, ß, or þ must have their CHAR-CODE in the range 0..255 - the implementation is free to use any code. But it does require that no #\Newline to CRLF translation takes place, among others.

Common Lisp has a long tradition of distinguishing character from byte (binary) I/O, e.g. READ-BYTE and READ-CHAR are in the standard. Some implementations let both functions be called interchangeably. Others allow either one or the other. (The simple stream proposal defines the notion of a bivalent stream where both are possible.)

Varying element-types are useful as some protocols rely on the ability to send 8-Bit output on a channel. E.g. with HTTP, the header is normally ASCII and ought to use CRLF as line terminators, whereas the body can have the MIME type application/octet-stream, where CRLF translation would destroy the data. (This is how the Netscape browser on MS-Windows destroys data sent by incorrectly configured Webservers which declare unknown files as having MIME type text/plain - the default in most Apache configurations).
What follows is a list of implementation dependent choices and behaviours and some code to experiment.

Here's some sample code to play with:
(defvar *unicode-test-file* "faithtest-out.txt")

(defun generate-256 (&key (filename *unicode-test-file*)
			  #+CLISP (charset 'charset:iso-8859-1)
  (let ((e (or external-format
	       #+CLISP (ext:make-encoding :charset charset :line-terminator :unix))))
    (describe e)
    (with-open-file (f filename :direction :output
		     :external-format e)
        (loop with s = (make-string 256)
	      for i from 0 to 255
	      do (setf (char s i) (code-char i))
	      finally (return s))
      (file-position f))))

;(generate-256 :external-format :default)
;#+CLISP (generate-256 :external-format :unix)
;#+CLISP (generate-256 :external-format 'charset:ascii)

(defun check-256 (&optional (filename *unicode-test-file*))
  (with-open-file (f filename :direction :input
		     :element-type '(unsigned-byte 8))
    (loop for i from 0
	  for c = (read-byte f nil nil)
	  while c
	  unless (= c i)
	  do (format t "~&Position ~D found ~D(#x~X)." i c c)
	  when (and (= i 33) (= c 32))
	  do (let ((c (read-byte f)))
	       (format t "~&Resync back 1 byte ~D(#x~X) - cause CRLF?." c c) ))
    (file-length f)))

(check-256 *unicode-test-file*)
(progn (generate-256 :external-format :unix) (check-256))
; uses UTF-8 -> 385 bytes

(progn (generate-256 :charset 'charset:iso-8859-1) (check-256))

(progn (generate-256 :external-format :default) (check-256))
; uses UTF-8 + CRLF(on MS-Windows) -> 387 bytes

(progn (generate-256 :external-format
  (ext:make-encoding :charset 'charset:iso-8859-1 :line-terminator :mac)) (check-256))
(progn (generate-256 :external-format
  (ext:make-encoding :charset 'charset:iso-8859-1 :line-terminator :dos)) (check-256))

Fast Bulk I/O

If you need to copy a lot of data and the source and destination are both streams (of the same element type), it's very fast to use READ-SEQUENCE and WRITE-SEQUENCE:
(let ((buf (make-array 4096 :element-type (stream-element-type input-stream)))
  (loop for pos = (read-sequence input-stream)
        while (plusp pos)
        do (write-sequence buf output-stream :end pos))))

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